High-speed delay scanning and deep learning techniques for spectroscopic SRS imaging

ABSTRACT

Systems and methods implement of high-speed delay scanning for spectroscopic SRS imaging characterized by scanning a first pulsed beam across a stepwise reflective surface (such as a stepwise mirror or a reflective blazed grating) in a Littrow configuration to generate near continuous temporal delays relative to a second pulsed beam. Systems and methods also implement deep learning techniques for image restoration of spectroscopic SRS images using a trained encoder-decoder convolution neural network (CNN) which in some embodiments may be designed as a spatial-spectral residual net (SS-ResNet) characterized by two parallel filters including a first convolution filter on the spatial domain and a second convolution filter on the spectral domain.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to and claims the benefit of U.S.Provisional Application No. 63/141,176, filed on Jan. 25, 2021 and U.S.Provisional Application No. 62/967,820, filed on Jan. 30, 2020, thecontents of which are incorporate herein in their entirety by reference.

STATEMENT OF GOVERNMENT SUPPORT

This invention was made with government support under Grant No. GM118471awarded by the National Institutes of Health. The government has certainrights in the invention.

TECHNICAL FIELD

The present disclosure relates to systems and methods for improvinglabel-free vibrational imaging by stimulated Raman scattering (SRS).Improvements include systems and methods for implementing high-speeddelay scanning (which may advantageously include tunable delay rangecapabilities) and systems and methods implementing computationaltechniques (including deep learning) for improving the signal-to-noiseratio (SNR) of spectroscopic images with both spectral and spatialdomains.

BACKGROUND

Stimulated Raman scattering (SRS) microscopy is a high-speed vibrationalimaging modality that is advantageously capable of producing chemicalmaps in dynamic living systems based on intrinsic molecular vibrations.This capability enables direct visualization of complex biologicalprocesses (without perturbation) as can be applied to a wide range ofbiomedical applications, including, e.g., tracking voltage spikingduring neuron firing, identifying the cancer margin of fresh,unprocessed tissues, and discovering biomarkers and therapeutic targetsof aggressive cancers. When evaluating an SRS system, speed, spectralbandwidth, and signal-to-noise ratio (SNR) are the three major aspects,which together characterize the temporal resolution, chemicalspecificity, and reliability of the system.

In SRS microscopy, a sample is coherently excited by photons from twolasers: one is pump photon with the frequency of ω_(P) and the other isa Stokes photon with the frequency of ω_(S). When the difference infrequency bet wen pump and stokes photons Δω=ω_(P)−ω_(S) is equal to aparticular Raman-active molecular vibration of the sample, SRS signalsequivalent to changes in the intensity of the pump and Stokes beams(including both stimulated Raman loss (SRL) and stimulated Raman gain(SRG)) are generated due to the nonlinear interaction between thephotons and the molecules. SRS imaging is free from the nonresonantbackground, exhibits identical spectrum as spontaneous Raman, and islinearly proportional to the concentration of the analyte, therebyallowing for quantification thereof.

In the original implementation of SRS microscopy, lasers with narrowspectral bandwidth were used to excite a single Raman-active vibrationalmode for fast imaging with high spectral resolution. Single-color (i.e.,non-stereoscopic) SRS is advantageously fast having reached nearreal-time speeds similar to video-rates. However, with a single-colorSRS excitation scheme, other vibrational modes of the sample are notexcited, thus failing to utilize the full advantage of the rich Ramanspectroscopic information and making it impossible to distinguish mixedchemical species with overlapped Raman bands in the sample. Forquantitative analysis, such as the concentration ratios betweendifferent chemical compounds, multicolor imaging with multiple chemicalcontrasts is highly desirable. Furthermore, simultaneous mapping ofdifferent chemical species in the same sample is extremely important forthe investigation of co-distribution or dynamic correlation betweenpairs of biomolecules in many in vivo biological and biomedicalapplications. In addition, since the cross-phase modulation signal andtwo-color two-photon absorption (TPA) from pigments or blood may existas a global non-Raman background in SRS, additional contrast away fromany Raman resonance may be required to remove the background for acutequantitative investigations.

Spectroscopic SRS was developed to acquire a Raman spectrum at eachpixel, enabling the simultaneous study of chemicals with overlappingRaman bands in complex biological samples. Spectroscopic SRS has beenachieved in several ways, including:

-   -   Spectral scanning of a narrowband laser such as disclosed in        Ozeki, Y. et al. High-speed molecular spectral imaging of tissue        with stimulated Raman scattering. Nat. Photonics 6,        845-851 (2012) and Zhang, D. et al. Quantitative vibrational        imaging by hyperspectral stimulated Raman scattering microscopy        and multivariate curve resolution analysis. Anal. Chem. 85,        98-106 (2013);    -   Parallel detection of a complete spectrum by a detector array        such as disclosed in Liao, C.-S. et al. Microsecond scale        vibrational spectroscopic imaging by multiplex stimulated Raman        scattering microscopy. Light Sci. Appl. 4, e265 (2015); and    -   Spectral focusing via temporal scanning of linearly chirped        pulses such as disclosed in Liao, C.-S. et al. Stimulated Raman        spectroscopic imaging by microsecond delay-line tuning. Optica        3, 1377-1380 (2016) (“Liao”). Spectral focusing is further        depicted and described with respect to FIG. 4 which depicts        linearly chirping pump and Stokes pulses and focus their entire        bandwidth into a narrow spectral region. In this way, each        temporal delay Δt between the chirped pulses corresponds to a        Raman shift ω.

Despite major advances in instrumentation that push the speed and thespectral bandwidth, most SRS applications are focused on thecarbon-hydrogen (C—H) stretching region (2800-3100 cm⁻¹) where strongRaman bands reside. However, the highly crowded SRS signals in the C—Hregion severely limit the chemical specificity of SRS in a complexbiological environment. Thus, recent SRS applications have explored theso-called fingerprint region (500-1800 cm⁻¹). Vibrational spectra in thefingerprint region display a higher degree of heterogeneity, which isimportant for differentiation of multiple species. Fingerprint regionSRS can therefore significantly enhance chemical specificity byproviding a specific and well-separated Raman spectrum for eachbiochemical component. However, the intensity of Raman signals in thefingerprint region is often lower as compared to the C—H region whichcan result in a high signal-to-noise ratio (SNR). To maintain highimaging speed, the laser power can be increased (within the damagethreshold of the sample) to prevent the signal from being overwhelmed bynoise. Thus, it is advantageous to provide a spectral acquisition schemewith high power efficiency. In addition to SNR considerations, Ramanpeaks for different biochemicals in the in the fingerprint region may benarrow and close to each other. Thus, it is further advantageous toprovide a spectral acquisition scheme with high spectral resolution.

To acquire high-fidelity fingerprint SRS spectra at the microsecondlevel, a high-speed spectral acquisition scheme that can achieve bothhigh power efficiency and high spectral resolution is desirable.Spectral acquisition schemes that utilize spectral focusing exhibit highpower-efficiency (since all the energy of the pulses is used). They arealso able to record a spectrum within a ˜200 cm-1 window at themicrosecond level (see Liao). However, existing implementations forhigh-speed spectral focusing using an edge-reflected resonant mirror(such as disclosed in Liao) typically exhibit a relatively small maximumdelay range (2 ps in Liao) with respect to temporal delays between pumpand the Stokes beam pulses thereby limiting the degree of chirping andleading to insufficient spectral resolution for the (28 cm⁻¹ spectralresolution in Liao). Thus, there exists a need for spectral acquisitionschemes that are able to acquire fingerprint SRS spectra at themicrosecond level with sufficient spectral resolution for thefingerprint region (e.g., spectral resolution below 10 cm⁻¹).

Due to the physical limits, advances of instrumentation alone are notenough to achieve reliable high-speed fingerprint spectroscopic SRSimaging. The physical limits lead to the trade-offs between speed,spectral bandwidth, and SNR, which can be conveniently expressed as a 3Dhyperplane design space (FIG. 1). Various computational methods havebeen proposed to extend the design space. Matrix completion (Lin, H.,Liao, C.-S., Wang, P., Kong, N. & Cheng, J.-X. Spectroscopic stimulatedRaman scattering imaging of highly dynamic specimens through matrixcompletion. Light Sci. Appl. 7, (2018) and Soldevila, F., Dong, J.,Tajahuerce, E., Gigan, S. & de Aguiar, H. B. Fast compressive Ramanbio-imaging via matrix completion. Optica 6, 341 (2019)) and compressedsensing (Romberg, J. Imaging via compressive sampling. IEEE SignalProcess. Mag. 25, 14-20 (2008) and Studer, V. et al. Compressivefluorescence microscopy for biological and hyperspectral imaging. Proc.Natl. Acad. Sci. 109, E1679-87 (2012)) methods have been used tosub-sample images to increase speed while avoiding information loss.Denoising algorithms with models on object structures (Dabov, K., Foi,A. & Katkovnik, V. Image denoising by sparse 3D transformation-domaincollaborative filtering. IEEE Trans. Image Process. 16, 2080-2095 (2007)and Buades, A., Coll, B. & Morel, J. M. A non-local algorithm for imagedenoising. Proc.—2005 IEEE Comput. Soc. Conf. Comput. Vis. PatternRecognition, CVPR 2005 II, 60-65 (2005)) have also been proposed torecover the SNR of microscopic images with low light exposure or lowpixel dwell times.

Most of these conventional computational methods depend on theformulation of forward models to describe the underlying imagingprocess, such as the modulation of measurements by a mask, the blurringof the image by the optical point-spread function, the thermal andelectronic noise of photodetector and the laser shot noise. However,formulating such forward models often requires tedious systemcalibration, and sacrifices by way of simplifications for the sake ofcomputational tractability. Thus, computational methods that can bypassmodel design and directly learn features of the image to formulatemappings from raw experimental data to reliable results shouldoutperform conventional computational methods such as described above.

Deep learning (Lecun, Y., Bengio, Y. & Hinton, G. Deep learning. Nature521, 436-444 (2015)) is one appealing approach that bypasses modeldesign and has the potential to extend the design space (as depicted inFIG. 1). Given training data of input/output image pairs, a deep neuralnetwork can learn nonlinear mappings that find optimal approximatesolutions to a variety of complicated inverse problems which wouldotherwise be challenging to address using conventional analyticalmethods. Deep learning has been applied in the context of somevibrational imaging applications, such as image restoration ofsingle-color SRS images in the C—H region under low light exposure(Manifold, B., Thomas, E., Francis, A. T., Hill, A. H. & Fu, D.Denoising of stimulated Raman scattering microscopy images via deeplearning. Biomed. Opt. Express 10, 3860 (2019)) and automated detectionof the tumor margin from fresh tissue (Zhang, L. et al. Rapid histologyof laryngeal squamous cell carcinoma with deep learning based stimulatedRaman scattering microscopy. Theranostics 9, 2541-2554 (2019)). However,there exists a need for computational methods that apply deep learningto processing spectroscopic images with both spectral and spatialdomains (e.g., as those spectroscopic SRS images).

Needs for better spectral acquisition schemes and better computationalmethods which enable, inter alia, improved spectroscopic imaging inspectral and spatial domains (including improved spectroscopic SRSimaging of the fingerprint region) are met by the systems and methodsdisclosed herein.

SUMMARY

In exemplary embodiments, a high-speed delay scanning assembly isdisclosed for imaging modalities that utilize varying temporal delaysbetween pulsed interrogation beams. The assembly may advantageouslyinclude a fast linear scanner and a stepwise reflective surface (e.g., astepwise mirror or a blazed grating), where the scanner is configured torepeatedly scan a first pulsed interrogation beam along a scan lineacross the stepwise reflective surface in a Littrow configuration. Thisscanning changing a path distance of the first beam (with each step ofthe stepwise reflective surface) thereby introducing a sequence ofvarying temporal delays relative to a second pulsed interrogation beam(which may advantageously have a constant beam path). In exampleembodiments, the scanner may be a polygon scanner, thereby enablingrapid and repeated linear scanning across the stepwise reflectivesurface. In some embodiments, the assembly may be used for spectroscopicSRS imaging (e.g., where the first pulsed interrogation beam is one of(i) a Stokes beam or (ii) a pump beam). In other embodiments, theassembly may be used for transient absorption spectroscopy (e.g., wherethe first pulsed interrogation beam is one of (i) a pump beam or (ii) aprobe beam). In yet further embodiments, the assembly may be used forImpulsive Stimulated Raman Scattering (ISRS) (e.g., where the firstpulsed interrogation beam is one of (i) a Stokes beam or (ii) a pumpbeam). In some embodiments, the stepwise reflective surface mayretroflect the first pulsed interrogation beam back along its originalpath, whereby the first interrogation beam is combined with the secondpulsed interrogation beam and linearly chirped by a high dispersionmedium (e.g., high dispersion glass rods) to temporally separatedifferent frequency components prior to sample interrogation. The firstand second pulsed interrogation beams may be generated by (i)femtosecond lasers, (ii) supercontinuum lasers, or (iii) broadbandlasers. In example embodiments, a maximum delay or delay range may betunable by rotating the scan line relative to the stepwise reflectivesurface thereby changing an angle between the scan line and a contourline of the stepwise reflective surface while still maintaining theLittrow configuration. Thus, in some embodiments, the maximum delay ordelay range may be selected to match pulse chirping of the first pulsedinterrogation beam. Advantageously the stepwise configuration mayprovide for high spectral linearity.

In further exemplary embodiments, improved techniques are disclosed forimage restoration of spectroscopic images with spectral and spatialdomains. This in example embodiments, a method for image restoration mayinclude (i) providing an encoder-decoder convolution neural network(CNN) characterized by an architecture that includes convolutionsinvolving both spectral and spatial domains; (ii) training the CNN witha pairs of raw and ground truth spectroscopic images which include bothspectral and spatial information, wherein training determines parametersfor the CNN that minimize a loss function between a CNN predictions andcorresponding ground truth; and (ii) applying the trained CNN to improvesignal to noise in raw images. In some embodiments, the CNN may be a 3DU-net network (e.g., that includes a 3×3×3 convolution filter). In otherembodiments, the CNN may be a spatial-spectral residual net (SS-ResNet)that includes two parallel filters including a first convolution filteron the spatial domain and a second convolution filter on the spectraldomain. Thus, for example, the CNN may include a parallel convolutionsublayer which includes a first convolution filter (with a (1,3,3)kernel size) for the spatial domain and a second convolution filter(with a (3,1,1,) kernel size) for the spectral domain. In exampleembodiments, image restoration may also include applying pixel-wiseLASSO unmixing to suppress the crosstalk between different chemical maps(such unmixing may advantageously be based upon a principle that at eachspatial location, only a few chemical components have dominantcontributions). Advantageously, the disclosed image restorationtechniques may be applied with respect to spectroscopic SRS images(including those in the fingerprint domain).

In further exemplary embodiments, a fast spectroscopic SRS imagingsystem is disclosed. The system may advantageously include both a highspeed scanning assembly (including a fast linear scanner and areflective blazed grating, wherein the scanner is configured torepeatedly scan a first pulsed interrogation beam along a scan lineacross the blazed grating in a Littrow configuration therebycontinuously changing a path distance of the first beam and introducinga substantially continuous sequence of varying temporal delays relativeto a second pulsed interrogation beam) and an image processor(configured to restore raw images based on application of a trainedencoder-decoder convolution neural network (CNN) characterized by anarchitecture that includes convolutions involving both spectral andspatial domains).

Any two or more of the features described in this specification,including in this summary section, may be combined to formimplementations not specifically described in this specification.

At least part of the techniques described in this specification may beconfigured or controlled by executing, on one or more processingdevices, instructions that are stored on one or more non-transitorymachine-readable storage media. Examples of non-transitorymachine-readable storage media include read-only memory, an optical diskdrive, memory disk drive, and random-access memory. At least part of thetechniques described in this specification may be configured orcontrolled using a computing system comprised of one or more processingdevices and memory storing instructions that are executable by the oneor more processing devices to perform various control operationsincluding high-current testing. The devices, systems, and/or componentsdescribed herein may be configured, for example through design,construction, arrangement, placement, programming, operation,activation, deactivation, and/or control.

The details of one or more implementations are set forth in theaccompanying drawings and the following description. Other features andadvantages will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 conceptually illustrates using deep learning to extend the designspace for spectroscopic SRS imaging, according to the presentdisclosure.

FIGS. 2 and 3 illustrate example embodiments for high-speed delayscanning, according to the present disclosure.

FIG. 4 illustrates spectral focusing according to the presentdisclosure.

FIG. 5 depicts a first example of an encoder-decoder CNN architecturefor SNR recovery of spectroscopic SRS images, according to the presentdisclosure.

FIG. 6 depicts a second example of an encoder-decoder CNN architecturefor SNR recovery of spectroscopic SRS images, according to the presentdisclosure.

FIG. 7 illustrates the process of SNR recovery and chemical mapping,according to the present disclosure.

FIGS. 8 and 9 provide validation analysis for an example application ofthe systems and methods of the present disclosure to imaging of lipidmetabolism in Mia PaCa-2 cells, according to the present disclosure.

FIGS. 10, 11 and 12 provide validation analysis for an exampleapplication of the systems and methods of the present disclosure tomapping biomolecules in a mouse brain, according to the presentdisclosure.

FIG. 13 provides validation analysis for an example application of thesystems and methods of the present disclosure to imaging of E. Colibiofuel production, according to the present disclosure.

FIG. 14 illustrates spectral resolution, spectral linearity, andsensitivity of high-speed delay scanning techniques, according to thepresent disclosure

FIG. 15 illustrates principles of transfer learning of a trainedencoder-decoder CNN.

DETAILED DESCRIPTION

Systems and methods are provided herein which, inter alia, improvespectroscopic SRS imaging. The systems and methods disclosedadvantageously enable high-fidelity fingerprint spectroscopic SRSimaging scheme with microsecond spectral acquisition. This capability isachieved by combined innovations of (i) novel configurations forhigh-speed delay scanning—characterized by scanning a first pulsed beam(e.g., the Stokes beam in SRS imaging) across a stepwise reflectivesurface (such as a stepwise mirror or a blazed grating) in a Littrowconfiguration to generate near continuous temporal delays relative to asecond pulsed beam (e.g., the pump beam in SRS imaging) and (ii) novelcomputational techniques for image restoration of spectroscopic imageswith both spectral and spatial domains (e.g., spectroscopic SRSimages)—including deep learning techniques using a trainedencoder-decoder convolution neural network (CNN) such as a U-net networkwhich in some embodiments may be designed as a spatial-spectral residualnet (SS-ResNet) characterized by two parallel filters including a firstconvolution filter on the spatial domain and a second convolution filteron the spectral domain. Advantages provided by the high-speed delayscanning configurations disclosed herein include greater delay range andspectral resolution relative to previously disclosed techniques forhigh-speed spectral focusing and, in some embodiments, delay rangetunability (such as to better enable matching delays to pulse chirping).Advantages provided by computational techniques disclosed herein includeimproved signal to noise ratio (e.g., reaching comparable quality toimages taken with two orders of magnitude longer pixel dwell times).

High-Speed Delay Scanning:

As described herein, the systems and methods of the present disclosuremay advantageously implement an improved high-speed delay scanningscheme. While example embodiments described herein relate tospectroscopic SRS imaging, the high-speed delay scanning schemedisclosed herein likewise applies to a broad range of modalities thatutilize a long delay scan (such as transient absorption spectroscopy andimpulsive SRS imaging). The high-speed delay scanning scheme disclosedherein includes a fast linear scanner (such as polygon scanner) and astepwise reflective surface (such as a stepwise mirror or a blazedgrating). The scanner is used (e.g., rotated in the case of a polygonalscanner) to continuously scan a pulsed beam along a scan line across thestepwise reflective surface in a Littrow configuration such that thescanned beam is perpendicular to the reflective surface (e.g.,perpendicular to the blazed surface of a blazed grating) whereby theincidence and diffraction angles for the scanned beam are identical andthe beam is retroflected along the same beam path. Optics such as acollimating lens may be included between the scanner and the blazedgrating to facilitate focusing the scanned beam in theLittrow-configuration relative to the blazed grating.

Advantageously, as the pulsed beam is scanned linearly along thestepwise reflective surface, the path distance between the scanner andthe surface changes with each step. Thus, scanning results in pulsesbeing reflected back along the same beam path with a varying temporaldelays. This can be used, for example, to introduce a repeating rapidsequence of changing beam-path differences between first and secondpulsed beams (e.g., between a Stokes beam and a pump beam in the case ofSRS imaging applications). In example embodiments, the maximum delayand/or delay range may be tunable by rotating the scan line relative tothe stepwise reflective surface (e.g., relative to a blazed surface)thereby changing an angle between the scan line and a contour line ofthe stepwise surface (e.g., between the scan line and a blazed line ofthe grating)—all while still maintaining the Littrow configuration. Suchrotation effectively changes the gradient of the scan line therebyallowing turning of the maximum delay and/or delay range.

In comparison to previous configurations (such as the polygonFourier-domain delay line configuration disclosed in Liao), thehigh-speed tunable delay scheme disclosed herein significantly improvesboth versatility and reliability. Advantageously, the high-speed tunabledelay scheme disclosed herein promotes linearity between the datasampling from the trigger and corresponding Raman shifts therebyminimizing distortion to the recorded spectrum. Moreover, since themaximum delay and/or delay range is tunable (e.g., by rotating theblazed grating relative to the scan line), the delay can be perfectlymatched relative to the pulse chirping.

Example embodiments are further described with respect to the figures.With initial reference to FIG. 2, an example configuration forhigh-speed delay scanner assembly 100 is depicted. The configuration ofFIG. 2 is advantageously adapted for spectroscopic SRS imaging. Thus,the delay scanner assembly 100 includes two chirped/pulsed beams (e.g.,generated by femtosecond lasers)—pump beam 102 and Stokes beam 104—whichare linearly chirped by high dispersion medium to temporally separatedifferent frequency components. As depicted, the Stokes beam 104 isreflected by a scanner which is depicted as a polygon scanner (PS) 106resulting in a repeating scan line 112, e.g., as PS 106 rotates (eachscan by the PS 106 thus introduces a continuous increase of light pathfor a few millimeters). In example embodiments, PS 106 may be a 55-kHzpolygon scanner. PS 106 reflects the Stokes beam 104 (via collimatinglens 108) whereby the beam is scanned across a blazed grating 110 (whichis configured in a Littrow configuration relative to the beam) along thescan line 112. As the beam is scanned, the beam path changes (as afunction of the slope 114 of the blazed grating 110). Thus, the blazedgrating 110 acts as a stepwise wedge to introduce a substantiallycontinuous-changing path difference between the pump beam 102 and theretroreflected Stokes beam 104. In alternative embodiments (notdepicted), the pump beam and the Stokes beam may be reversed so that achirp/pulse sequence of varying delays is introduced with respect to thepump beam relative to the Stokes beam (instead of to the Stokes beamrelative to the pump beam as depicted in FIG. 2). The disclosedconfiguration enables fast acquisition of an SRS spectrum (e.g., within20 μs).

Advantageously, the delay scanner assembly 100 may be configured toenable tuning of the delay range 116. For example, the blazed grating114 may be rotatable (e.g., about an axis perpendicular to the plane ofthe blazed surface) to enable changing an angle θ 118 between the scanline 112 and a grating blazed line 120. As depicted, reducing the angleθ shortens the effective delay range (e.g., by reducing scan distancethat is in line with the slope 114) while increasing the angle θlengthens the effective delay range (e.g., by increasing scan distancethat is in line with the slope 114). In example embodiments, the tunabledelay rage may be adjustable from 0˜20 ps. Maximum delay range may bedetermined by length of the scan line and the blazed angle of thegrating 110. The ability of the delay scanner assembly 100 to providefor a large delay range and fast chirping has the potential todrastically improve spectral resolution while maintaining fast speeds.

FIG. 3 further illustrates example features of the delay scannerassembly 100 of FIG. 2. With reference to both FIGS. 2 and 3, a moredetailed example of optical setup for delay scanner assembly 100 isdescribed. In example embodiments (such as depicted in FIG. 3), the pumpbeam 102 and the Stokes beam 104 may be implemented using a dual-output80-MHz femtosecond pulsed laser 122 (InSight DeepSee+, Spectra-Physics).By way of example, a 120-fs tunable laser (680-1300 nm) may be used forthe pump beam 102 and a 200-fs output fixed at 1040 nm may be used asthe Stokes beam. In some embodiments (such as depicted in FIG. 3), theStokes beam 104 may first be modulated by an acousto-optical modulator(AOM) 124 (1205-C, Isomet) at 2.4 MHz for heterodyne detection. Next theStokes beam 104 passes through a polarizing beam splitter (PBS) 126 andquarter-wave plate (QWP) 128 prior to scanning (as described herein)using PS 106 (e.g., Lincoln SA24, Cambridge Technology) and blazedgrating 114. Each scan by PS 106 introduces a near continuous increaseof light path for a few millimeters, resulting in a series of nearcontinuous temporal delays between the pump and the retroreflectedStokes beam. The retroflected Stokes beam 104 passes back through theQWP and to the PBS—which due to polarization caused by the Stokes beam104 having passed in both directions through the QWP now reflects theretroflected Stokes beam 104 (thereby separating it from continuing backalong the original beam path). The retroflected Stokes beam 104continues through a half-wave plate (HWP) (e.g., to match polarizationwith the pump beam 10). Next, the retroflected stokes beam 104 iscombined along the same path as the pump beam 102 using a dichroicmirror (DM) 130 and both beams pass through high dispersion medium whichtemporally separates out different frequency components. In exampleembodiments, both beams may be broadened, e.g., to picosecond.Preferably, high dispersion glass rods may be used for broadening thebeams (such as SF57 with 90 cm in total length).

It is understood that the detailed example of optical setup for delayscanner assembly 100 depicted and described with respect to FIGS. 2 and3 is not limiting. Rather, as would be appreciated by person skilled inthe art, any number of different optical configurations may be utilizedfor achieving spectral focusing. In some embodiments (e.g., depending onrest parameters for specific end applications) the optical configurationmay include various polarization elements, filters, or other featuresfor imparting specific characteristics to the pump and/or Stokes beams.Generally, systems and methods may utilize a scanner 106 and blazedgrating 110 to impart near continuous temporal delays between the pumpand Stokes beam for any number of different spectroscopic SRS opticalconfigurations. As discussed herein, this includes exampleimplementations where the scanner 106 and blazed grating 110 are used toimpart near continuous temporal delays to the pump beam rather than theStokes beam.

With continued reference to FIGS. 2 and 3, after broadening, thechirped/pulsed beams are sent collinearly to an upright microscope 200for spectroscopic SRS imaging (e.g., in the fingerprint region). Anexample configuration for microscope 200 is depicted in greater detailin FIG. 3. Microscope 200 advantageously provides for sampleinterrogation, signal detection and signal processing. While exampleconfigurations for microscope 200 are described herein, it isappreciated that any number of other spectroscopic SRS imagingconfigurations may be utilized.

In the depicted embodiment, sample interrogation for Microscope 200includes image scanning by way of a 2-D galvo scanner set (GVS102,Thorlabs) characterized by galvo mirrors (GM) 202. Sample interrogationfor microscope 200 further include various beam focusing elements (e.g.,lens assembly 204) and an objective OBJ 206 for focusing the light ontothe sample 208 (such as a 60×, 1.2 NA water immersion objective (UPLSASP60 W, Olympus)). It is appreciated that the present disclosure is notlimited to 2-D image scanning. Rather any number of differentconfigurations may be utilized to provide for image scanning across oneor more spatial dimensions. In example embodiments the microscope 200may provide for 3D image scanning (e.g., tomographic, label-freemolecular imaging). This may be achieved, e.g., by combining principalsfor spectroscopic optical coherence tomography (SOCT) with SRS toachieve SRS-SOCT.

Microscope 200 further provides for signal detection and processing.Thus, in the depicted embodiment, an oil immersion condenser (C) 210 maybe utilized for forward collection. The Stokes beam may then be filtered(using filter (F) 212. SRS signals may be optically collected, e.g.,using a photodiode (PD) 214 (such as S3994-01, Hamamatsu) with acustom-built resonant circuit. SRS signals may then be extracted, e.g.,using a lock-in amplifier (LIA) 216 (such as UHFLI, Zurich Instrument)and digitize, e.g., by a high-speed digitizer (such as ATS 460,AlazarTech). In example embodiments, a custom-written Matlab (MathWorks)code was used to synchronize the scanning of spectrum with the polygonscanner and the scanning of the galvo mirrors to generate thespectroscopic image stack, which may be a 3D mixed domain image stack(λ,X,Y) including wavelength A and 2D spatial coordinates X and Y. It isappreciated that in other embodiments image scanning across 3 spatialdimensions may result in the spectroscopic image stack being a 4D mixeddomain image stack (λ,X,Y,Z) including wavelength λ and 3D spatialcoordinates X, Y, and Z. In some embodiments, spatial coordinates may berepresented in a different coordinate system (e.g., in polar coordinatesinstead of cartesian coordinates).

While the example configuration of FIGS. 2 and 3 is adapted forspectroscopic SRS imaging, it is appreciated that the describedhigh-speed delay scanner assembly 100 depicted and described may beadopted for other modalities that similarly utilize a long delay scan.Example applications include, inter alia, transient absorptionspectroscopy and impulsive SRS imaging. A brief discussion of theseexample applications is included below:

-   -   In transient absorption spectroscopy, a fraction of the        molecules is promoted to an electronically excited state by        means of an excitation (or pump) pulse. A probe pulse (which        typically has low intensity to avoid multiphoton/multistep        processes during probing) is sent through the sample with a        delay with respect to the pump pulse. A change in the absorption        spectrum is calculated, i.e., as the difference between the        absorption spectrum of the excited sample and the absorption        spectrum of the sample in a ground state. By changing the time        delay between the pump and the probe and recording a change in        the absorption spectrum at each time delay a profile can be        obtained as a function of both delay and wavelength. This        profile advantageously may contain information on the dynamic        processes occurring in the sample (e.g., in a photosynthetic        system under study). Example processes that can be analyzed        include excited-state energy migration, electron and/or proton        transfer processes, isomerization, and intersystem crossing.        Advantageously, the high-speed delay scanner assembly 100 of        FIGS. 2 and 3 may be adapted to provide for continuously        changing the time delay between pump and probe pulsed beams in        transient absorption spectroscopy. In particular, the scanner        106 and blazed grating 110 may be utilized (in a similar manner        as described with respect to FIGS. 2 and 3) to continuously        change the beam path of either the pump or probe pulsed beams.    -   In Impulsive Stimulated Raman Scattering (ISRS) applications,        vibrational oscillations stimulated by a Raman pulse (RP) (e.g.,        coherently stimulated by a femtosecond Raman pulse) are        monitored (e.g., in real time) and interrogated as intensity        modulations in the transmission of a temporally delayed probe        pulse (PP). ISRS is another powerful technique which is        advantageously able to monitor (e.g., in the time-domain)        vibrational fingerprints of a sample. ISRS may typically use        femtosecond broadband pulses for stimulation and probing. The        two temporally separated chirped/pulsed laser fields RP and PP        are exploited to stimulate and read out vibrational signatures        in the sample. When the RP is shorter than the period of a        normal mode, it can generate a localized wave-packet that        coherently oscillates and evolves as a function of time. The        photo-excited wave-packet modulates the transmissivity of the        sample at the frequencies of the stimulated Raman modes, which        can thereafter be detected by monitoring the PP transmission (as        a function of both temporal delay between RP and PP pulsed and        wavelength (where Fourier transformation over the temporal delay        yields the Raman spectrum of the system of interest).        Advantageously, the high-speed delay scanner assembly 100 of        FIGS. 2 and 3 may be adapted to provide for continuously        changing the time delay between RP and PP pulsed beams in ISRS.        In particular, the scanner 106 and blazed grating 110 may be        utilized (in a similar manner as described with respect to FIGS.        2 and 3) to continuously change the beam path of either the RP        or PP pulsed beams.

Compared with previous spectral focusing implementations (such asdisclosed in Liao which maxed out a 12-kHz resonant scanner), thehigh-speed delay scanning scheme disclosed herein drastically improvesspeed (by 5-fold) while also achieving higher spectral linearity andresolution. In test implementations, the spectral coverage was ˜200 cm⁻¹(primarily due to the spectral bandwidth of the laser sources. However,since the delay range is freely tunable by rotating the blazed grating,this could allow for combination with broadband lasers (e.g., usingfiber amplification; Alonso-Gutierrez, J. et al. Metabolic engineeringof Escherichia coli for limonene and perillyl alcohol production. Metab.Eng. 19, 33-41 (2013)) or even with supercontinuum laser sources. Thus,it is anticipated that the high-speed delay scanning scheme may enableobtaining the entire fingerprint SRS spectrum within 20 μs.

FIG. 14 illustrates spectral resolution, spectral linearity, andsensitivity of example embodiments of the high-speed delay scanningscheme disclosed herein (e.g., with respect to FIGS. 2 and 3). Referringto FIG. 14, advantageously, a longer delay range enables the use of90-cm SF57 glass rods after the beam combiner, resulting in a spectralresolution of 10 cm⁻¹ in the fingerprint region (a) as compared to only3 cm⁻¹ for non-SRS spontaneous Raman spectroscopy (b). Such spectralresolution is essential for resolving multiple chemicals in afingerprint window. In addition, given the linear speed of the polygonscanner, the acquired raw Raman spectrum is free of spectral channeldistortion. For evaluation, the spectral profiles of five chemicals wasmeasured (c) and compared with spontaneous Raman spectroscopy (d).Eleven significant peaks were used to map (e) Raman shifts samplingpoint numbers of the digitizer (corresponding to acquisition time fromsampling trigger). Mapping showed a high linearity with R²=0.9997. Thesensitivity was quantified by acquiring SRS spectra from dimethylsulfoxide (DMSO) diluted with DI water to different concentrations (f).Besides the background due to cross-phase modulation, the DMSO solutionscontributed to a significant peak at 2913 cm⁻¹. At concentrations as lowas 0.125% v/v, the DMSO peak was still separable from the background,suggesting a high sensitivity in the C—H region. However, in thefingerprint region, excessive averaging is necessary to obtain an SRSspectrum with high SNR. Fingerprint SRS spectra for adenine by singleacquisition (g) includes a large amount of noise versus the by 100averaging (h).

Image Restoration of Spectroscopic SRS Images:

The systems and methods of the present disclosure may alsoadvantageously implement improved computational techniques forrestoration of spectroscopic images with spectral and spatial domains(e.g., for improving the SNR of raw SRS spectroscopic images obtainedfor the fingerprint region using the high-speed delay scanning schemedescribed herein). These improved computational techniques may includedeep learning using a trained encoder-decoder CNN such as a U-netnetwork. Typically, encoder-decoder CNNs are developed for processingimages in either two (X,Y) or three spatial dimensions (X,Y,Z).Consider, for example, the U-net network structure originally developedfor processing images in three spatial dimension (X,Y,Z) (Ronneberger,O., Fischer, P. & Brox, T. U-Net: Convolutional Networks for BiomedicalImage Segmentation in International Conference on Medical imagecomputing and computer-assisted intervention 234-241 (2015)(“Ronneberger”). According to the present disclosure encoder-decoderCNNs originally developed for processing spatial domain image stacks maynow be adapted for processing mixed spectral and spatial domain 3D imagestacks (such as for spectroscopic SRS). This represents the first timeanyone has applied U-Net (or similar encoder-decoder architecture) toprocessing spectroscopic SRS images.

Encoder-decoder CNNs refer to U-net networks and other similararchitectures that include both a contracting (encoder) path and anexpansive (decoder) path (u-shaped architectures). The contracting pathis a typically a convolutional network that consists of repeatedapplication of convolutions, each followed by a rectified linear unit(ReLU) and a max pooling operation. During the contraction, the domaininformation is reduced while feature information is increased. Theexpansive pathway combines the feature and domain information through asequence of up-convolutions and concatenations with high-resolutionfeatures from the contracting path. U-Net (as used herein) refers bothto the Ronneberger disclosed architecture and other adaptations thereof.Psuedo-3D residual networks have also been developed for learningspatio-temporal video representation (Qiu, Z., Yao, T. & Mei, T.Learning Spatio-Temporal Representation with Pseudo-3D ResidualNetworks. Proc. IEEE Int. Conf. Comput. Vis. 2017-Octob, 5534-5542(2017) (“Qiu”)).

While direct application of a 3D encoder-decoder CNN (e.g., a 3D U-netnetwork) to spectroscopic SRS images demonstrates significant SNRimprovement there are some shortcomings. SRS images are unique (relativeto other typical 3D image stacks) in that they include spectral domainfeatures as one of the dimensions (and are therefore different than 3Dimage stacks containing volumetric data). Conventional 3Dencoder-decoder architectures fail to consider the different physicalcorrelations of spatial and spectral domain, which may introduceartifacts and degrade recovery quality. In addition, training a deepnetwork with 3D CNN filters may be difficult and has a high computationcost. Thus, systems and methods disclosed herein also provide anencoder-decoder network with improved convolution filtering designedspecifically to handle mixed spectral and spatial domain spectroscopicimages. This further facilitates deep learning as a practical tool forfingerprint spectroscopic SRS system (with a much higher speeds andgreater spectral fidelity level than conventional SRS imaging). In someembodiments, improved convolution filtering may be implemented in aspatial-spectral residual network (SS-ResNet) characterized byconvolution layers that employ two parallel filters (similar to thespatio-temporal network in Qiu) including a first convolution filter inthe spatial domain and a second convolution filter in the spectraldomain. Thus, instead of utilizing a 3×3×3 3D CNN filter, systems andmethods of the present disclosure may utilize: (1) A 1×3×3 convolutionfilter on the spatial domain to capture spatial correlations; and (2) A3×1×1 convolution filter on the spectral domain to maintain spectralcontinuity between adjacent frames. SS-ResNet reduces the training modelsize, which facilitates the training of a deep network. Moreimportantly, spatial-spectral crosstalk distortions may advantageouslybe avoided, which improves the reconstruction accuracy (compared toconventional 3D CNN).

In further example embodiments SS-ResNet may be expanding to handle a 4Dmixed domain Spectrographic image stack, e.g., (λ,X,Y,Z) includingwavelength λ and 3D spatial coordinates X, Y, and Z. This may be usefulfor SRS-SOCT and other SRS imaging with 3D spatial scanning. In suchembodiments, SS-ResNet may again be characterized by convolution layersthat employ parallel filters including a first convolution filter in thespatial domain and a second convolution filter in the spectral domain.In example embodiments this may include: (1) A 1×3×3×3 convolutionfilter on the spatial domain to capture spatial correlations; and (2) A3×1×1×1 convolution filter on the spectral domain to maintain spectralcontinuity between adjacent frames. Alternatively, each 2D pairing inthe spatial domain (X,Y), (X,Z), (Y,Z) may be processed independentlyusing (1) A 1×3×3 convolution filter on the spatial domain to capturespatial correlations; and (2) A 3×1×1 convolution filter on the spectraldomain to maintain spectral continuity between adjacent frames.

Additional computational advantages are also provided by someembodiments employing a pixel-wise least absolute shrinkage andselection operator (LASSO) regression algorithm (such as disclosed inTibshirani, R. Regression Shrinkage and Selection via the Lasso. J. R.Stat. Soc. Ser. B 58, 267-288 (1996)) may be adapted to decompose therecovered spectroscopic image into maps of different biomolecules.Compared to conventional unmixing by least-square fitting (such asdisclosed in Ruckebusch, C. & Blanchet, L. Multivariate curveresolution: A review of advanced and tailored applications andchallenges. Anal. Chim. Acta 765, 28-36 (2013)) the disclosedapplication of pixel-wise LASSO unmixing may advantageously suppress thecrosstalk between different chemical maps by incorporating priorknowledge that only a few components have dominant contributions at eachlocation.

With reference to FIG. 7, in some embodiments a two-step processingapproach involves SNR recovery and chemical mapping may be applied toextract information from high-speed yet noisy spectroscopic images. Torecover the SNR, a deep neural network may be employed, acting as asupervised denoiser, to recover the SNR of high-speed fingerprint SRSimages. First, pairs of spectroscopic SRS images are generated as thetraining set (Step 1), with high-speed, low SNR images as the rawacquisition and a low-speed, high-SNR image (through averaging ofmultiple raw acquisitions) as the ground truth. After training data isacquired, the data is used for training a spatial-spectral residual net(SS-ResNet) deep neural network, as described herein (Step 2). Trainednetwork is then applied to recover the SNR of high-speed yet noisyimages (Step 3). Advantageously, up-sampling and skip-connection layersin the network improves the resolution of learned features and thusrequires less training samples. Residual learning may be applied tofacilitate training (He, K., Zhang, X., Ren, S. & Sun, J. Deep residuallearning for image recognition in Proceedings of the IEEE conference oncomputer vision and pattern recognition 770-778 (2016)). After SNRrecovery, the spectroscopic image stack may be linearly decomposed intochemical maps (Step 4) to facilitate downstream visualization andanalysis. Based on the observation that at each spatial location, only afew chemical components have dominant contributions, pixel-wise LASSOregression may be utilized, which incorporates individual l₁-normsparsity regularization to the concentrations at each pixel. Notably,the level of regularization can be fine-tuned such that the output cansuppress cross-talks between different channels while avoidingartifacts.

In example embodiments, a trained network may also be quickly tweaked todenoise other samples by transfer learning. As shown in FIG. 15, anetwork pre-trained on Mia PaCa-2 cells was applied to recover prostatetissue images taken under the same imaging conditions. Directapplication achieved high SNR levels but sacrificed spatial resolutiondue to the differences between spatial features for the two datasets. Byfeeding in training data of the new samples, the network required lessthan half of the training epochs to converge and output high-resolution,high-SNR images, making it convenient to apply to differentapplications.

Deep Learning:

Example embodiments of the systems and methods disclosed herein arefurther described with respect to the figures. With reference now toFIG. 5, a first example of an encoder-decoder CNN architecture isdepicted for SNR recovery of spectroscopic images with both spectral andspatial domains (e.g., spectroscopic SRS images). In particular, FIG. 5depicts the main layers of the encoder-decoder CNN. The examplearchitecture of FIG. 5 is based on a U-net network (3 level) andincludes an encoder phase 510 and a decoder phase 520.

Each level of the encoder phase 510 includes 3D convolution layer(s)(with (3,3,3) kernel size). Levels of the encoder phase 510 areseparated by a ReLU/maxpooling layer (with a (2,2,2) kernel size). Eachlevel of the decoder phase 510 similarly includes 3D convolutionlayer(s) (with (3,3,3) kernel size). Levels of the decoder phase 520 areseparated by (i) an up-sampling layer (with a (2,2,2) kernel size) forup-sampling the feature map and (ii) a concatenation layer forconcatenation the up-sampled feature map with the corresponding featuremap from the encoder phase.

The U-net network may further include a final convolution stage 530(with a (1,1,1) kernel size) used to map the feature maps into theprediction of pixel values of the high SNR image (e.g., in testembodiments 32-channel feature vectors were mapped into the residualsbetween ground truth (GT) and input). This prediction (i.e., theresiduals) may be added with the initial input to yield the processedhigh-SNR image (such that the prediction value is residual with respectto the raw input image) (He, K., Zhang, X., Ren, S. & Sun, J. Deepresidual learning for image recognition in Proceedings of the IEEEconference on computer vision and pattern recognition 770-778 (2016)).

With reference now to FIG. 6 (a and b), a further example of anencoder-decoder CNN architecture is depicted for SNR recovery ofspectroscopic images with both spectral and spatial domains (e.g.,spectroscopic SRS images). The encoder-decoder CNN of FIG. 6 implementsan example embodiment of SS-ResNet characterized by inclusion ofspatial-spectral convolution layers (referred to herein as SS-Conv 615),each of which feature parallel convolution filters for spatial andspectral domains). FIG. 6(a) depicts the main layers of theencoder-decoder CNN while FIG. 6(b) depicts sublayers for each SS-Conv615 of the encoder-decoder CNN (including convolution sublayersfeaturing parallel convolution filters for spatial and spectraldomains).

Like other embodiments, the example of FIG. 6 employs an encoder-decoderarchitecture (3 level) with an encoder phase 610 and a decoder phase620. As depicted in FIG. 6, Each level of the encoder phase 610 mayinclude the following convolution layers: (i) an initial convolutionlayer (with a (1,1,1) kernel size) which increases the featuredimensions and (ii) a SS-Conv layer 615 (which may be cycled severaltimes, e.g., 6 times per the depicted embodiment). Note that lowestlevel (level 3) includes the addition of a final convolution layer (witha (1,1,1) kernel size). Levels of the encoder phase 610 are separated bya ReLU/max-pooling layer (with a (2,2,2) kernel size) to reduce thedimensions. Each level of the decoder phase 620 may include thefollowing convolution layers: (i) an initial convolution layer (with a(1,1,1) kernel size), (ii) a SS-Conv layer 615 (which, as with thedecoder phase, may be cycled several times, e.g., 6 times per thedepicted embodiment) and (iii) a final convolution layer (with a (1,1,1)kernel size). Levels of the decoder phase are separated by (i) anup-sampling layer (with a (2,2,2) kernel size) and (ii) a concatenationlayer. As with the embodiment of FIG. 5, the end prediction (i.e., theresidual) may be added with the initial input to yield the processedhigh-SNR image.

With specific reference to FIG. 6(b), detailed architecture of theSS-Conv layer 615 is depicted. In particular, each SS-Conv layer 615,includes a parallel convolution sublayer which includes a firstconvolution filter (with a (1,3,3) kernel size) for the spatial domainand a second convolution filter (with a (3,1,1,) kernel size) for thespectral domain. The parallel convolution sublayer is followed by aconcatenation sublayer and an additional convolution sublayer layer(with a (1,1,1) kernel size) to reduce channel features—the result ofwhich is added to the input and either gets cycled through the SS-Convlayer 615 again or proceeds to the next level.

As noted above, it is appreciated that the embodiment of FIG. 6 may beexpanded to processing stereoscopic SRS images with 3D spatial data. Insuch embodiments, parallel filters may include, e.g., a firstconvolution filter (with a (1,3,3,3) kernel size) for the spatial domainand a second convolution filter (with a (3,1,1,1) kernel size) for thespectral domain. Alternatively, each 2D pairing in the spatial domainmay be processed and reconstructed independently (e.g., producingoptimized image data for each of (X,Y), (X,Z), (Y,Z) subdomains).

Encoder-decoder CNNs such as described herein may be trained using atraining set that includes raw and a ground truth (GT) image sets.During training, parameters may be learned by minimizing a loss function(e.g., a loss function that averages the mean squared error and/orutilizes a structural similarity index) between the prediction (based onthe raw images) and ground truth. In example embodiments, the U-netnetwork may be implemented using Keras and trained using a graphicsprocessing unit (GPU, RTX 2080 Ti, Nvidia). To quantify thereconstruction error, the ground truth and predicted image may first benormalized (e.g., using the technique described Weigert, M. et al.Content-aware image restoration: pushing the limits of fluorescencemicroscopy. Nat. Methods 15, 1090-1097 (2018)) after which errormeasurements may be calculated, e.g., normalized root-mean-square errorNRMSE and/or structural similarity index measure (SSIM).

Pixel-Wise LASSO Unmixing:

Advantageously, systems and methods of the present disclosure maypixel-wise LASSO unmixing to suppress the crosstalk between differentchemical maps (e.g., by incorporating prior knowledge that only a fewcomponents have dominant contributions at each location). LASSO haspreviously been used to solve problems in which the variable is sparse,e.g., compressed sensing (Candès, E. J. & Wakin, M. B. An introductionto compressive sampling. IEEE Signal Process. Mag. 25, 21-30 (2008)).With the use of LASSO unmixing, it is possible to resolve more chemicalsin the same window since LASSO due to crosstalk suppression betweendifferent channels. An example application of pixel wise LASSO isdescribed in the paragraphs that follow:

Assuming the dimensions of the spectroscopic SRS image in x,y,λ asN_(x), N_(y), N_(λ), the image stack may be rearranged as a 2D datamatrix (D∈

^(N) ^(x) ^(N) ^(y) ^(×N) ^(λ) ) by arranging the pixels in the rasterorder. Given the number of pure components as K, a bilinear model may beused to decompose the data matrix into the multiplication ofconcentration maps C∈

^(N) ^(x) ^(N) ^(y) ^(×K) and spectral profiles of pure chemicals S∈

^(K×N) ^(λ) :D=CS+E  (1)

Where E is the residual term. To simplify the problem, S may be obtainedby measuring the spectral profiles from pure chemicals. Theconcentrations can be obtained by minimizing the error term E throughthe least-squares fitting. However, in practice, least-squares fittingalone generates chemical maps with severe cross-talks in complexbiological samples where many biochemicals have overlapping spectralprofiles. To improve the performance, it can be observed that for eachspatial pixel, only a few chemical components contribute significantly,which is equivalent to the sparsity of concentrations at each pixel.Thus, an l₁ norm regularization may be introduced to the originalleast-squares fitting problem, leading to the following optimizationproblem which solves for the optimal solution Ĉ:

$\begin{matrix}{{\hat{C} = {\arg{\min\limits_{C}\{ {{\frac{1}{2}{{D - {CS}}}^{2}} + {\beta{\sum\limits_{i = 1}^{N_{x}N_{y}}{C_{i,;}}}}} \}}}},} & (2)\end{matrix}$

where β is a hyper-parameter controlling the level of the sparsity ofthe concentration maps and C_(i) ∈

^(K) is the vector containing all the concentration values at a spatialpixel location. For a set of data recorded in the same imaging anddigitizing conditions, the value of β needs tuning only once.

Example Applications:

To demonstrate the advantages of the systems and methods of the presentdisclosure stereoscopic high-speed SRS imaging was performed for severalscenarios using a combination of the high-speed delay scanning andcomputational techniques disclosed herein. Scenarios tested include:

-   -   real-time imaging of lipid species, including cholesterol and        unsaturated fatty acids, in living cancer cells.    -   large-area mapping of biomolecules in the mouse whole brain,        revealing distinctive distributions of fatty acid and        cholesterol in nerve bundles and populations of cholesterol-rich        cells in certain brain regions.    -   differentiating multiple biomolecules by imaging biofuel        production by engineered microbes.

These applications and the results disclosed herein collectivelydemonstrate the ability of the systems and methods of the presentdisclosure to perform high-speed, high-fidelity fingerprintspectroscopic SRS imaging and its potential in addressing a plethora ofsignificant biomedical and bioengineering problems.

Imaging of Lipid Metabolism in Mia PaCa-2 Cells:

Lipid metabolism is a cellular process involving spatiotemporal dynamicsof fatty acid and cholesterol. The distributions of different lipidspecies in the cell are tightly regulated to ensure proper cellularactivities and function. Abnormal lipid metabolism is related to manyhuman diseases including aggressive cancer. Thus, quantitative imagingof lipids in living systems is of great interest. Unlike fluorescenceimaging by lipophilic dyes, Raman spectroscopy provides high chemicalspecificity to differentiate lipid species, such as cholesterol andvarious fatty acids. With enhanced signal levels, SRS is capable ofquantitative imaging of specific lipid species. For example, cholesterolimaging has been demonstrated in cholesterol-rich samples such as theatherosclerotic artery and lysosome-related organelles in C. elegans byfocusing on the sterol C═C stretching band at 1669 cm⁻¹. However, due tothe limited signal levels in the fingerprint region, except in theabovementioned cases of excessive accumulation, it remains challengingto study cholesterol in living cells or large-area tissues.

For the purposes of this example application, Mia PaCa-2 cancer cellswere grown in a monolayer at 37° C. in 5% CO₂ in RPMI-1640 mediumsupplemented with 10% fetal bovine serum. To prepare fixed cell samplesfor training, Mia PaCa-2 cells were cultured on a glass-bottom dish for1-2 days at the humidified chamber and were fixed with 10% neutralbuffered formalin for 15 minutes at room temperature. The cells werethen washed with and imaged in PBS buffer. For cholesterol depletion inMia PaCa-2 cells, 500 μM HPβCD was added to the medium and cultured for24 hr.

To demonstrate real-time lipid tracking in living cells, Mia PaCa-2cells were imaged using the systems and methods described herein withinthe 1550-1750 cm⁻¹ fingerprint vibrational window. For training, adataset was acquired consisting of pairs of raw and ground truth imagesof Mia PaCa-2 cells. Fixed Mia PaCa-2 cells were used to ensure that theground truth images (formulated by excessive averaging) did not sufferfrom motion artifacts. Each raw spectroscopic image stack covering a˜200 cm⁻¹ spectral window with 200×200 μm2 field-of-view (FOV) wasacquired within 1.8 seconds. The ground truth image was generated byaveraging 100 raw images of the same FOV, resulting in a ˜10-fold SNRenhancement. After training, the performance of SNR recovery wasvalidated using a set of previously unseen images. FIG. 8 compares theraw (a), SS-ResNet network recovered (b) and ground truth (GT) (c)images of the same FOV at 1650 cm⁻¹. This demonstrates that theSS-ResNet network recovery allows reconstruction of the rawspectroscopic image stack, reaching comparable image quality to theground truth images. FIG. 8 compares the raw (a), SS-ResNet networkrecovered (b) and ground truth (GT) (c) images of the same FOV at 1650cm⁻¹. This demonstrates that the SS-ResNet network recovery allowsreconstruction of the raw spectroscopic image stack, reaching comparableimage quality to the ground truth images.

To test whether the network recovery facilitates downstream spectralanalysis, a small region of interest from the validation set wasselected. Referring to FIG. 8, A zoom in comparison of this region ofinterest shown in each of dashed boxes in (a)-(c) is depicted in (d).Again SS-Res-Net can be seen to perform well compared to GT. This regionof interest was used to perform pixel-wise LASSO unmixing on raw,SS-ResNet and ground truth image stacks using three SRS spectralprofiles generated from Bovine serum albumin (BSA), triglyceride andcholesterol. These spectral profiles represent 3 major chemical bonds,including the Amide I band at 1650 cm⁻¹ from proteins, the acyl C═C bandfrom lipid acyl chains at 1650 cm⁻¹, and the sterol C═C band fromcholesterol a 1669 cm⁻¹. Referring again to FIG. 8, fingerprint SRSspectra of BSA, cholesterol and triglyceride is depicted in (e) servingas spectral references for protein, cholesterol and unsaturated fattyacid. Chemical maps of protein, cholesterol and fatty acid by pixel-wiseLASSO unmixing are shown in (f). A quantitative analysis of the chemicalmapping accuracy after network recovery is shown in (g) providing theSSIM for raw (vs. GT) and network (vs. GT) of the three chemicalchannels. The outputs from the network and the ground truth showedsimilar spatial distributions and concentrations for all 3 components.In contrast, the results from the raw data failed to provide insightsinto the distributions of chemical species and were difficult todistinguish from the background noise. To quantify the quality ofchemical maps after network recovery, the SSIM index was calculated forall the three chemical channels. The SSIM indices increased considerablyafter recovery, which proved that the approach did not introduceartifacts and provided reliable results on the subsequent chemicalanalysis.

To apply this high-speed, high-sensitivity technique to the real-timemapping of lipid in living cells, high-resolution images of living MiaPaCa-2 cells were recovered from the raw images taken at high speed byapplying the same SS-ResNet trained on fixed cells. In living Mia PaCa-2cells, lipid droplets are shown to be highly dynamic. Live-cell imagingat the speed of 1.8 seconds per stack was performed on Mia PaCa-2 cellsto capture lipid droplet dynamics (resulting in the observation ofsevere motion artifacts in the 100-averaged image from the live-celldata). SS-ResNet recovered images from a single frame showed clearcircular-shaped droplets within the cells, highlighting the importanceof temporal resolution during live-cell imaging. The chemical maps ofcholesterol and fatty acid further confirmed that motion artifactsaffect the fidelity of the subsequent spectral analysis. After recovery,clear lipid dynamics can be visualized at 1650 cm′ and real-timechemical mapping of protein, cholesterol and fatty acid can be achieved.With reference to FIG. 8, SS-ResNet recovery of raw single acquisitionand 100 averaging is compared for SRS imaging of living Mia PaCa-2 cells(h), and cholesterol (i) and fatty acid (j) maps by LASSO unmixing.Three significant motion artifacts are highlighted.

It was also investigated whether the systems and methods of the presentdisclosure could be used to track changes in cholesterol amount anddistribution. To that end, two sets of living Mia PaCa-2 cells wereimage: a control set and a set treated with HPβCD, which extractscholesterol from the cell membrane. Compared with the control group, thecholesterol concentration in the cell membrane decreased significantlyafter HPβCD treatment, whereas the fatty acid concentration wasmaintained at the same level. The remaining cholesterol after HPβCDtreatment mainly distributed within the lipid droplets. By calculatingthe single-cell ratio between cholesterol and fatty acid concentrationsfor ˜1000 cells from the control and the HPβCD-treated groups,significant reductions in cellular cholesterol after the treatment wereconfirmed. These data show that deep-learning high-speed fingerprint SRSimaging enables high-fidelity, real-time chemical mappings of chemicalbonds in single living cells and facilitates the tracking of metabolitedynamics at subcellular levels. Referring to FIG. 8, high-speed imagingand chemical maps of protein, cholesterol and fatty acid of living MiaPaCa-2 cells are depicted in normal (control) conditions (k-l) and withHPβCD treatment (m-n), thereby enabling a single-cell statisticalanalysis of the ratio between cholesterol and fatty acid over ˜1000cells in control and HPβCD treated group (o).

FIG. 9, illustrates the relative performance of an SS-ResNet network (e)as compared to raw images (a), ground truth (GT) (b), block-matching 4Dfiltering (BM4D) (which is a state-of-the-art unsupervised 3D imagedenoising algorithm) (c), and a 3D CNN network (d). Each was trained andtested on the same dataset. The results indicate that both SS-ResNet and3D CNN networks outperformed BM4D significantly. Meanwhile, theSS-ResNet network performed better than the 3D CNN by maintaining moredetailed structures without introducing artifacts. These Observationswere quantified by calculating the normalized root mean square error(NRMSE) and structural similarity (SSIM) index versus GT for each ofraw, BM4D, 3D CNN and SS-ResNet reconstructions (f-g). Both measurementssuggest significant improvement of the image quality using SS-ResNet.

Mapping of Biomolecules in a Mouse Brain:

Brain tissue is comprised of many cell types, and biomolecules in thetissue are highly heterogeneous among different brain areas. Chemicalmapping of the whole brain is essential for studying the functionalityof molecules in the brain. Previous label-free metabolic studies ofmouse whole brain slices were mainly based on multi-color SRS imaging inthe C—H window, providing only protein and lipid information. For thesake of maintaining sample conditions during the experiment, the totalacquisition time of a mouse whole brain slice is usually several hours.Therefore, it remains challenging to perform spectroscopic SRS imagingin the fingerprint region to generate chemical maps of otherbiomolecules.

For the purposes of this example application, a mouse brain slice wasprepared from a mouse (Jackson Lab) at age 21 days. PBS was used forperfusion, after which formalin was perfused to fix the brain tissue.Then the brain tissue was frozen sectioned at 150 μm thickness.

Following the procedures in FIG. 7, a training dataset was generated fordifferent brain regions, including the lateral hypothalamus (LH),caudate putamen (CPu), cortex (CTX), habenula (HB), medial habenula(MH), ventral lateral nucleus (VL), hippocampus (HC), dentate gyrus (DG)and corpus callosum (CC). Each raw image was taken at a speed of 3.8seconds per spectroscopic image stack with a 200×200 μμmm2 FOV and thehigh-SNR ground truth GT image was acquired by averaging the rawmeasurements of the same FOV 100 times. The whole brain training setincluding GT and downstream chemical maps by LASSA unmixing isillustrated in FIG. 10. After training, a validation set was used totest the ability to recover SNR using SS-ResNet. The results aredepicted in FIG. 11. After recovery, the SNR of the raw image improvedsignificantly while the subcellular details are preserved, reachingcomparable image quality to the ground truth image (see FIG. 11(a-c)comparing raw, SS-ResNet and GT, respectively). To quantify thereconstruction quality, the NRMSE and SSIM verses GT for raw andSS-ResNet were calculated (FIG. 11(d)). Taking advantage of the highimaging speed and the ability to recover high SNR by SS-ResNet,fingerprint SRS spectroscopic imaging was performed on a mousewhole-brain slice. Acquisition of the whole brain slice over a ˜200 cm-1spectral window in the fingerprint region was finished within 3.5 h,which is comparable to the acquisition time of multi-color SRS imagingin the C—H region focusing on a few Raman shifts. FIG. 12 illustratesthe comparison between the raw image and the network recovered the imageof the whole brain tissue at 1650 cm-1 and demonstrates thatmorphologies of single cells and nerve bundles within the brain can beclearly distinguished after recovery.

Referring again to FIG. 11, Pixel-wise LASSO spectral analysis of theSS-ResNet recovered image stack was also applied to produce chemicalmaps of the amide I group (protein, (e)), acyl C═C (unsaturated fattyacid (f)) and sterol C═C (cholesterol (g)). A composite image (h) of the3 components shows significant heterogeneity among different cells andbrain structures, reflecting a relative abundance of protein, fatty acidand cholesterol. To further characterize the distribution of thebiomolecules, analysis focused on several brain regions and features(i). Overall, the soma of mature neurons shows relatively lowerconcentrations of all three components compared to the surroundingtissue. Surprisingly, an abundant cholesterol-rich cells was foundpresent near neurons in the LH and basal amygdaloid (BM) regions, whichmay represent different metabolic activities in this population ofcells. It was also observed that nerve bundles in the ventral posteriornucleus (VP) and CPu are comprised of different ratios of cholesteroland fatty acid. Interestingly, there are a few rare cells that containhigh cholesterol concentrations in the DG region (Circled regions inFIG. 11(i)). As DG is one of the regions containing neuralstem/progenitor cells, it is suspected that these cholesterol-rich cellsmay reflect cells undergoing hippocampal neurogenesis. In summary, thelarge-area imaging in the fingerprint region is a viable tool for thelabel-free study of the cellular cholesterol content, which could beused to address many important biomedical questions about therelationship between cholesterol metabolic activity and a variety ofbrain diseases and disorders, including neurodegenerating disorders andbrain tumors.

Imaging of E. Coli Biofuel Production:

Limonene and pinene are biofuel precursors that can be producedbiosynthetically in microbes such as Escherichia coli (E. coli) usingstrains that have been engineered to produce the enzymes necessary tosynthesize these chemicals. Currently, quantitation of biochemicalproduction levels mainly relies on gas chromatography-mass spectrometry(GC-MS), which suffers from low throughput and requires extraction stepsthat destroy the sample. Strain engineering and optimization typicallyinvolve the construction of many variants, followed by screening, in alengthy iterative process. The limited throughput of GC-MS approacheshinders efficient optimization of design variables for biochemicalsynthesis. In addition, GC-MS only provides quantification ofpopulation-level production, ignoring the potential for genetic orphenotypic variation among cells. Thus, a high-throughput quantificationmethod that provides direct measurement of biofuel concentrations hasthe potential to improve the design, build, and test cycle necessary forimproving production strains. SRS is a promising approach to fulfillthis requirement by detecting intrinsic vibrational signatures from thebiofuels that are linearly related to the concentrations. Yet, due tothe overwhelming SRS contributions from endogenous proteins and lipids,quantitative imaging of the production levels for certain biofuels(i.e., limonene, pinene) in the crowded C—H region has been challenging.High-throughput SRS imaging in the fingerprint region is expected toaddress this challenge by providing specific and well-separated Ramanspectra for the biofuels.

E. coli strains used in this analysis are derived from strain JW0451-2(K-12 BW25113 ΔacrB) from the Keio collection. The kanamycin resistancemarker gene was removed from the Keio collection strain. This “wildtype” strain was then transformed with plasmids expressing theheterologous pathways for either pinene or limonene production. Forpinene production, the chassis strain was transformed with two plasmids,pJBEI-3933 & pJBEI-3085. For limonene production, the chassis strain wastransformed with plasmid pJBEI-640937, provided by Taek Soon Lee viaAddgene (#47048).

Prior to SRS imaging, overnight cultures were inoculated in LuriaBertani (LB) medium with appropriate antibiotics for plasmid maintenanceand refreshed the following day in 5 mL of M9 minimal media supplementedwith 20 g/L glucose and appropriate antibiotics. When the culturesreached an OD600 (optical density at 600 nm) of 0.6, pinene or limoneneproduction was induced by adding IPTG to the culture (500 μM and 25 μM,respectively). The cultures were grown at 37° C. for another 18-24hours. 5-10 minutes before imaging, 5 μL of culture was placed on a 3%agarose pad and pressed between microscope coverslips to immobilize thecells, and then the sample was imaged.

The systems and methods of the present disclosure provide a viableapproach towards high-throughput quantitative chemical imaging ofchemical compounds produced biosynthetically by bacteria. FIG. 13illustrates analysis results relating to such imaging. Chemical maps aredepicted in (a) for a ˜1650 cm⁻¹ fingerprint Raman window, including (i)unsaturated fatty acid (BSA) (peak at 1655 cm⁻¹), limonene (two peaks at1645 cm⁻¹ and 1678 cm⁻¹) and pinene (peak at 1660 cm⁻¹) The peaks alloriginate from C═C bonds but differ from each other due to the specificstructures of each chemical. Additionally, the amide 1 group fromprotein contributes a broad Raman band around 1650 cm⁻¹, serving as thecontrast for the cell body. Training and testing sets were acquired fromboth wild type cells and biofuel production strains, which consisted ofpairs of high-speed, low SNR and low-speed, high-SNR images through 50averages. After training, SS-ResNet was applied to a validation set totest the recovery performance. Examples of validation images at 1650cm⁻¹, including the raw image, SS-ResNet recovery (labeled as U-net) andground truth (GT) are shown in (b-d), respectively. Quantitation of thereconstruction quality is depicted in (e), suggesting that it ispossible to denoise images while maintaining high-quality spatiallocalization data. Finally, high-speed imaging and SS-ResNet recoverywas performed on images of a wild type strain (E. coli BW25113), whichdoes not produce biofuel (f). This was compared to limonene production(g) and pinene production (h) strains of E. coli. Based on the spectralprofiles from pure chemicals, pixel-wise LASSO spectral analysisdecomposed the network-recovered spectroscopic images of the strainsinto the maps of the three chemicals. The chemical maps indicated thatthe wild type strain only had significant signals from the protein inthe cell bodies. Whereas the limonene and pinene producing strains hadprotein signals and a substantial increase in the correspondingconcentrations of intracellularly aggregated chemicals. Using thesystems and methods of the present disclosure, the acquisition time offingerprint SRS imaging was 8 seconds for a 50×50 μm² FOV coveringhundreds of E. coli cells, offering excellent potential forhigh-throughput screening to optimize the design variables of biofuelproduction pathways.

Computer Implements:

All or part of the systems and methods described in this specificationand their various modifications may be configured or controlled at leastin part by one or more computers using one or more computer programstangibly embodied in one or more information carriers, such as in one ormore non-transitory machine-readable storage media. A computer programcan be written in any form of programming language, including compiledor interpreted languages, and it can be deployed in any form, includingas a stand-alone program or as a module, part, subroutine, or other unitsuitable for use in a computing environment. A computer program can bedeployed to be executed on one computer or on multiple computers at onesite or distributed across multiple sites and interconnected by anetwork.

Actions associated with configuring or controlling high-speed delayscanning or performing deep learning operations can be performed by oneor more programmable processors executing one or more computer programsto control or to perform all or some of the operations described herein.All or part of the imaging systems can be configured or controlled byspecial purpose logic circuitry, such as, an FPGA (field programmablegate array) and/or an ASIC (application-specific integrated circuit) orembedded microprocessor(s) localized to the instrument hardware.

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only storagearea or a random-access storage area or both. Elements of a computerinclude one or more processors for executing instructions and one ormore storage area devices for storing instructions and data. Generally,a computer will also include, or be operatively coupled to receive datafrom, or transfer data to, or both, one or more machine-readable storagemedia, such as mass storage devices for storing data, such as magnetic,magneto-optical disks, or optical disks. Non-transitory machine-readablestorage media suitable for embodying computer program instructions anddata include all forms of non-volatile storage area, including by way ofexample, semiconductor storage area devices, such as EPROM (erasableprogrammable read-only memory), EEPROM (electrically erasableprogrammable read-only memory), and flash storage area devices; magneticdisks, such as internal hard disks or removable disks; magneto-opticaldisks; and CD-ROM (compact disc read-only memory) and DVD-ROM (digitalversatile disc read-only memory).

Elements of different implementations described may be combined to formother implementations not specifically set forth previously. Elementsmay be left out of the systems described previously without adverselyaffecting their operation or the operation of the system in general.Furthermore, various separate elements may be combined into one or moreindividual elements to perform the functions described in thisspecification.

The invention claimed is:
 1. A high-speed delay scanning assembly forimaging modalities that utilize varying temporal delays between pulsedinterrogation beams, the assembly comprising a fast linear scanner and astepwise reflective surface, wherein the scanner is configured torepeatedly scan a first pulsed interrogation beam along a scan lineacross the stepwise reflective surface in a Littrow configurationthereby changing a path distance of the first beam and introducing asequence of varying temporal delays relative to a second pulsedinterrogation beam.
 2. The assembly of claim 1, wherein the scanner is apolygon scanner.
 3. The assembly of claim 1, wherein the second pulsedinterrogation beam has a constant beam path.
 4. The assembly of claim 1,wherein the assembly is for spectroscopic SRS imaging wherein the firstpulsed interrogation beam is one of (i) a Stokes beam or (ii) a pumpbeam.
 5. The assembly of claim 1, wherein the assembly is for transientabsorption spectroscopy wherein the first pulsed interrogation beam isone of (i) a pump beam or (ii) a probe beam.
 6. The assembly of claim 1,wherein the assembly is for Impulsive Stimulated Raman Scattering (ISRS)wherein the first pulsed interrogation beam is one of (i) a Stokes beamor (ii) a pump beam.
 7. The assembly of claim 1, wherein the stepwisereflective surface retroflects the first pulsed interrogation beam backalong its original path, whereby the first interrogation beam iscombined with the second pulsed interrogation beam and linearly chirpedby a high dispersion medium to temporally separate different frequencycomponents prior to sample interrogation.
 8. The assembly of claim 7,wherein the dispersion medium includes high dispersion glass rods. 9.The assembly of claim 1, wherein the first and second pulsedinterrogation beams are generated by (i) femtosecond lasers, (ii)supercontinuum lasers, or (iii) broadband lasers.
 10. The assembly ofclaim 1, wherein a maximum delay or delay range is tunable by rotatingthe scan line relative to the stepwise reflective surface therebychanging an angle between the scan line and a contour line of thestepwise reflective surface while still maintaining the Littrowconfiguration.
 11. The assembly of claim 1, wherein the maximum delay ordelay range is selected to match pulse chirping of the first pulsedinterrogation beam.
 12. The assembly of claim 1, wherein the assembly isconfigured to provide for high spectral linearity.
 13. A fastspectroscopic SRS imaging system, the system comprising: a high-speeddelay scanning assembly including a fast linear scanner and a reflectiveblazed grating, wherein the scanner is configured to repeatedly scan afirst pulsed interrogation beam along a scan line across the blazedgrating in a Littrow configuration thereby continuously changing a pathdistance of the first beam and introducing a substantially continuoussequence of varying temporal delays relative to a second pulsedinterrogation beam; and an image processor configured to restore rawimages based on application of a trained encoder-decoder convolutionneural network (CNN) characterized by an architecture that includesconvolutions involving both spectral and spatial domains.